The present invention relates to the use of code division multiple access (CDMA) communication techniques and cellular radio telephone communication systems, and more particularly, to a method and apparatus for rate detection of a transmitted signal for direct sequence-code division multiple access (DS-CDMA) communication techniques.
CDMA is a multiple access method, which is based on a spread spectrum modulation technique, that has in recent years been applied to commercial cellular radio systems, in addition to the previously used methods of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). Currently, channel access in cellular systems is commonly achieved using FDMA and TDMA. In an FDMA system, a communication channel is formed by a signal radio frequency band in which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters that only pass signal energy within the specified frequency band. Thus, as each channel is assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of time slots in a periodic frame of time slots which divide a frequency in one or more time. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of guard bands and a time gate or other synchronization element that only passes signal energy received at the proper time.
With FDMA, TDMA systems or hybrid FDMA/TDMA systems, one goal is to ensure that two strong, interfering signals do not occupy the same frequency at the same time. In contrast, CDMA allows signals to overlap in both time and frequency. Thus, CDMA signals share the same frequency spectrum in present day systems. In the frequency or the time domain, the multiple access signals appear to be “on top of” each other.
In CDMA, the narrow-band data signal of the user is modulated by a pseudo random sequence, called the spreading code, having a broader band than the data signal. In connection with modulation, the signal spreads to a relatively wide band. For example in known CDMA systems, bandwidths such as 1.25 MHz, 10 MHz and 50 MHz have been used.
The spreading code consists of a number of bits and the bit rate of the spreading code is much higher than that of the data signal. The bits of the spreading code are called chips in order to distinguish them from data bits and data symbols. Each data symbol of the user is multiplied by the chips of the spreading code. Thus, the narrow-band data signal spreads to the frequency band to be used. The ratio between the bit rate of the spreading code and the bit rate of the data signal is called the spreading ratio of the CDMA system.
In some systems, each connection has a unique, orthogonal spreading code so that the data signals of several users can be transmitted simultaneously on the same frequency band without mutual interference. Correlations are performed to extract desired signals from the receiver. The receivers then restore the band of the signal to its original bandwidth. Signals arriving at the receiver and containing spreading codes associated with other receivers do not correlate in an ideal case, but retain their wide band and thus appear as noise in the receivers. The spreading codes used by such systems are preferably selected in such a way that they are mutually orthogonal, i.e., they do not correlate with each other, as for example, Walsh codes.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be several times that of existing analog technology as a result of the properties of a wideband CDMA system, such as improved interference diversity, voice activity gating, and reuse of the same spectrum in interference diversity.
In order to transmit a voice signal, the signal is converted from analog to digital and then encoded by a speech coder. According to the U.S. standard entitled “Mobile Station Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems”, commonly known as IS-95, which is hereby incorporated by reference, a speech coder for transmission of voice signals uses a variable rate to encode the voice signals that are to be transmitted via the CDMA system. Four different information bit rates are possible in systems designed in accordance with this standard. The signals may additionally be encoded in order to facilitate error detection and correction, caused by transmitting the signal, through the use of a checksum, such as a cyclical redundancy check (CRC), block coding, convolutional coding, or combinations of all three. According to the IS-95 system standard the signals that are to be transmitted are convolutionally encoded using a ½ rate coder.
In convolutional coding, information bits are encoded and decoded in such a way as to determine information contained in bits which may have been destroyed in transmission. A convolutional code is typically described by the rate of the code, its constraint length, and various parity equations. If a ½ convolutional coder is used, then the output of the convolutional coder, for each 20 ms frame, typically has one of four sizes, for example, 384 bits, 192 bits, 96 bits, or 48 bits, corresponding to one of the four different possible rates of operation for the speech encoder, referred to as rates 1, 2, 3, and 4 respectively. In order to obtain the same number of encoded output bits for each of the different data rates, the data bits are repeated at different rates by a variable rate repeat coder, for example, in order to achieve 384 bits per frame. In order to keep the bit energy constant, the powers of the transmitted bits are scaled by factors of 1, ½, ¼, and ⅛ corresponding to the four different rates respectively. In addition, CRC check bits may be included for rates 1 and 2 (corresponding to 384 bits and 192 bits) but are not used at the lower rates 3 and 4.
In order to save overhead in the number of transmitted bits, no information or flag bits regarding the rate at which the voice signal was encoded by the speech coder are transmitted with the signals. This in turns leads to a significant problem for the receiver which must then determine at which of the four rates was the received voice signal encoded so that the receiver can properly decode the received data frames. This problem is commonly known in the industry as blind rate detection.
Previously at least two different solutions to the problem of blind rate detection of the received encoded signals have been proposed in association with the IS-95 system standard. According to one method, as seen in the IS-95 specification, the received signal is decoded four times, i.e., once each assuming a different one of the four possible data rates. The decoded signal is then re-encoded for all four rates and the number of positions with errors in the re-encoded signal and the received signal are compared. The CRC information is then used (for rates 1 and 2) in addition to the number of errors to make a decision regarding which of the assumed rates is correct. Another solution has been proposed whereby attempts to determine the rate are made by identifying patterns of repeated bits from the four rates, as described in “Multi-rate detection for the IS-95 CDMA forward traffic channels,” by E. Cohen et al., IEEE 1995 Global Telecommunications Conference (GLOBECOM '95), Singapore, Nov. 13-17, 1995.
Though each of the above proposed solutions are conceptionally simple, both solutions have inherent disadvantages. The first solution requires a considerable amount of fine tuning and the determination of many thresholds to work properly. As a result, a considerable amount of unnecessary processing takes place since the received signal is processed four times. The second solution is also problematic because its performance requires considerable additional processing. In addition, the accuracy in determining the correct rate according to this second solution is less than optimal. Therefore, there is a need for an improved means of blind rate detection and decoding for use with CDMA systems.